Vinylimidazole-based asymmetric ion pair comonomers: Synthesis, polymerization studies and formation of ionically crosslinked PMMA

Article abstract of DOI:10.1002/pola.26720

Vinylimidazole-based asymmetric ion pair comonomers (IPCs) which are free from nonpolymerizable counter ions have been synthesized, characterized and polymerized by free radical polymerization (FRP), atom transfer radical polymerization (ATRP), and revers

Full text of DOI:10.1002/pola.26720

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Vinylimidazole-Based Asymmetric Ion Pair Comonomers: Synthesis,
Polymerization Studies and Formation of Ionically Crosslinked PMMA
Satyasankar Jana,1 Vivek Arjunan Vasantha,1 Ludger Paul Stubbs,1 Anbanandam Parthiban,1
and Julius G. Vancso1,2
1Institute of Chemical and Engineering Sciences (ICES), Agency for Science, Technology and Research (A*STAR), 1 Pesek Road,
Jurong Island, Singapore 627833, Singapore
2MESA1 Research Institute for Nanotechnology, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500,
AE Enschede, The Netherlands
Correspondence to: S. Jana (E-mail: satyasankar_jana@ices.a-star.edu.sg) or A. Parthiban (E-mail: a_parthiban@ices.a-star.edu.sg)
Received 12 March 2013; accepted 5 April 2013; published online 15 May 2013
DOI: 10.1002/pola.26720
ABSTRACT: Vinylimidazole-based asymmetric ion pair comono-
mers (IPCs) which are free from nonpolymerizable counter ions
have been synthesized, characterized and polymerized by free
radical polymerization (FRP), atom transfer radical polymeriza-
tion (ATRP), and reversible addition-fragmentation chain trans-
fer (RAFT) mediated polymerizations in solution and by
dispersion polymerization in water. The asymmetric nature of
IPCs is due to the fact that cationic component of these IPCs is
derived from vinylimidazole (VIm) and anionic component is
derived from either styrenesulfonate (SS) or 2-acrylamido-2-
methyl-1-propanesulfonate. Although under ATRP, conversions
are either very low or negligible, FRP and RAFT produces poly-
mers with high to moderate monomer conversions but with
different solubility characteristics. This investigation provides
insight to the polymerization behavior of each component of
the asymmetric IPCs and also its effects on composition and
solubility characteristics of the resulting polymers. The IPCs
studied here are high temperature ionic liquid and thus the
polymers synthesized from these IPCs are highly ionic in na-
ture and possess very strong intermolecular interactions which
makes some of these IPC based polymers completely insoluble
in organic and aqueous solvents. This highly ionic interaction
is exploited to synthesize ionically crosslinked PMMA. MMA on
copolymerization with 5–6 mol % of IPC yielded copolymer
which is insoluble in common organic solvents like THF, DMF,
C
V
etc., unlike homo PMMA.
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KEYWORDS: ATRP; copolymrization; ion pair comonomer; ionic
liquid; polyampholyte; reversible addition-fragmentation chain
transfer (RAFT); styrenesulfonate; vinylimidazole
recently reported to exhibit anti-biofouling characteristics.6
The earliest synthesis and polymerization of IPCs were
reported by Salamone et al.7–10 To date, the IPCs that have
been studied well and available in the literature are (meth)a-
INTRODUCTION Ion pair comonomers (IPCs) are salts, made
up of both polymerizable cationic and anionic units and are
useful for the synthesis of ampholytic polymers or polyam-
pholytes. Polyampholytes1–3 are different from zwitterionic
polymers.4,5 In the case of polyampholytes, counterions
belong to different repeating unit of the polymer chain,
whereas the counter ions in zwitterionic polymers belong to
the same repeating unit. Generally, polymerization of IPCs
composed of cationic and anionic components of similar
reactivity produces charge neutral polyampholytes, which
have shown exceptional behavior with respect to their solu-
bility, viscosity, and so forth.1–3 Such polyampholytes are
potential materials for protein separation, binding metal ions
to separate it from crude oil in petroleum industry, and so
forth2 and could be an alternative to zwitterionic polymers.
Polyampholytes which are synthesized from IPC, have been
crylamide-(meth)acrylamide,8,9
(meth)acrylate-(meth)acry-
late,6,9–11 and 4-vinylpyridine-styrenesulfonate7 based
systems. We hereby report the synthesis, characterization
and polymerization behavior of vinylimidazole (VIm) based
asymmetric IPCs. We have chosen 1-vinylimidazole (VIm) as
a cationic component for the IPCs due to the significant in-
terest in VIm based poly(ionic liquids) (PILs) for many appli-
cations12–18 which include highly conductive polymers, use
as solvents for inorganic matrixes like graphene and carbon
nanotubes, chromatographic separations, enhanced CO2
absorption, and so forth. Similarly, sulfonate-based compo-
nent was chosen as the anionic component of IPCs because
Additional Supporting Information may be found in the online version of this article.
C
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of its wider application as polyelectrolyte, surfactant and
also in ion exchange resins. The polymerization behavior of
these IPCs was investigated in detail by free radical poly-
merization (FRP), atom transfer radical polymerization
(ATRP), and reversible addition-fragmentation chain transfer
(RAFT) mediated polymerizations. Controlled radical poly-
merization techniques were used here as the polymerization
techniques like ATRP,19,20 single-electron transfer living radi-
cal polymerization (SET-LRP),21 and RAFT20,22 can be
expected to provide greater control over the structure of the
polymers synthesized and also for the purpose of verifying
the suitability of these techniques for surface initiated poly-
merizations23 of asymmetric IPCs. Aqueous dispersion poly-
merization using free radical initiators was also used to
produce copolymers. Finally, the highly ionic interactions of
these IPC-based copolymers have been exploited to synthe-
size ionically crosslinked PMMA. Such copolymers of PMMA
even with 5–6 mol % IPC content yielded insoluble poly-
mers. The swelling studies of these copolymers which are
crosslinked by ionic interactions are also described herein.
60 C for 48 h. The mixture was cooled down to room tem-
perature, and the bottom layer was separated out and
washed with THF further (2 3 25 mL). The brownish liquid
product 3-hexyl-1-vinylimidazolium bromide (C6VImBr) was
then dried in a vacuum oven. Yield 14.22 g (51.63%). 1H
NMR (400 MHz, d4-MeOD, d): 0.92 (t, 3H, ACH3), 1.34–1.41
(m, 6H, 33CH2), 1.94 (t, 2H, CH2), 4.28 (t, 2H, NACH2), 5.45
(d, 1H, vinyl CH2), 5.96 (d, 1H, vinyl CH2), 7.28 (t, 1H, vinyl
CH), 7.79 (s, 1H, imidazole NACH), 8.02 (s, 1H, imidazole
CHAN1), 9.3 (s, reduced intensity, imidazole NACHAN1).
13C NMR (400 MHz, d4-MeOD, d): 14.29, 23.49, 26.96, 30.92,
32.28, 51.28, 109.96, 120.79, 124.45, 129.83.
Step II: Aqueous solution (50 mL) of 3-hexyl-1-vinylimidazo-
lium bromide (C6VImBr, 14.22 g, 54.9 mmol) was added to
an aqueous solution (100 mL) of sodium 4-styrenesulfonate
(SSS, 13.575 g, 65.04 mmol) and the homogeneous mixture
was stirred at room temperature overnight. The ion pair
monomer product was extracted from this homogeneous so-
lution using CHCl3 (3 3 75 mL). The chloroform phase was
then dried over anhydrous MgSO4 and removed in a rotary
evaporator followed by drying in a vacuum oven at room
temperature finally to yield white powder which was stored
in a freezer before use. Yield 19.5 g, 98%. The product was
soluble in MeOH, CHCl3, acetone, THF, DMF, CAN and insolu-
ble in toluene and water. 1H NMR (400 MHz, d4-MeOD, d):
0.92 (t, 3H, ACH3), 1.33–1.36 (m, 6H, 33CH2), 1.89 (t, 2H,
CH2), 4.22 (t, 2H, NACH2), 5.32 (d, 1H, styrene vinyl CH2),
5.42 (d, 1H, imidazole vinyl CH2), 5.83–5.92 (m, 2H, vinyl
CH2 of imidazole and styrene), 6.73–6.80 (m, 1H, styrene
vinyl CH), 7.19–7.25 (m, 1H, imidazole vinyl CH), 7.48 (d,
2H, styrene aromatic CH), 7.73 (d, 1H, imidazole NACH),
7.78 (d, 2H, styrene aromatic CH), 7.97 (d, 1H, imidazole
CHAN1), 9.29 (s, reduced intensity, imidazole NACHAN1).
13C NMR (400 MHz, d4-MeOD, d): 14.30, 23.48, 26.95, 30.89,
32.27, 51.23, 109.86, 115.90, 120.71, 124.40, 127.07, 127.28,
129.81, 137.27, 140.88, 145.67. Anal. calcd. For
EXPERIMENTAL
Materials and Characterizations
All chemicals including RAFT agents were purchased from
Sigma-Aldrich and Alfa Aesar. 2,20-Azobisisobutyronitrile
(AIBN) was obtained from HalloChem Pharm Co. AIBN was
purified by recrystallization from absolute alcohol. Spectra/
Por regenerated cellulose membrane tubing for dialysis
(MWCO 1000 and 3500) was bought from Spectrum Labs.
NMR spectra were recorded on a 400 MHz Bruker Ultra-
Shield AVANCE 400SB spectrometer. Elemental microanalysis
was performed using Eurovector E300 elemental analyzer.
Thermogravimetric analysis (TGA) was performed under N2
ꢀ
atmosphere at a heating rate of 10 C/min using SDT-2960T
TA instruments. The differential scanning calorimetry (DSC)
was performed using Perkin Elmer Pyris Diamond Hyper DSC
instruments at a heating rate of 10 ꢀC/min under N2. The
aqueous GPC system was equipped with a Delta 600 HPLC
pump, a 600 controller, a 717 plus autosampler, a 2487 dual
absorbance detector, and a 2414 refractive-index detector, all
from Waters. The following GPC columns were arranged in se-
ries: Ultrahydragel guard, and Ultrahydragel 120 (7.8 mm ID
3 300 mm) and an Ultrahydragel Linear (7.8 mm ID 3 300
mm). The eluant (0.1 M NaNO3 in deionized water) flow rate
was 0.7 mL/min and the columns were maintained at 30 ꢀC.
The results were obtained using polyethylene oxide (PEO) and
polyethylene glycol (PEG) calibrations.
C
19H26N2O3S: C 62.96, H 7.33, N 7.73, S 8.85, O 13.24%;
Found: C 62.26, H 7.10, N 7.62, S 8.41, O 12.91%.
Synthesis of IPC2
This novel comonomer was synthesized by a modified
method of Qiu et al.24 in a two-step procedure as given
below and stored in freezer before use.
Step I: Excess 1-bromohexadecane (38.93 g, 127.5 mmol)
was added to a solution of 1-vinylimidazole (VIm, 8 g, 85
mmol) in dry THF (80 mL) in a round bottom flask fitted
ꢀ
with condenser and the solution was heated at 60 C for 48
h. The homogeneous solution was then concentrated to ꢁ50
mL and the product was precipitated from excess ether (120
mL). The white product 3-hexadecyl-1-vinylimidazolium bro-
mide (C16VImBr) was then dried in a vacuum oven at room
temperature to produce white powder. Yield 15.25 g, 45%.
1H NMR (400 MHz, d4-MeOD, d): 0.91 (t, 3H, ACH3), 1.30-
1.39 (m, 26H, 133CH2), 1.94 (t, 2H, CH2), 4.28 (t, 2H,
NACH2), 5.47 (d, 1H, vinyl CH2), 5.96 (d, 1H, vinyl CH2),
7.28 (t, 1H, vinyl CH), 7.79 (s, 1H, imidazole NACHAN1),
Synthesis of Vinylimidazole (VIm) Based IPCs
Synthesis of IPC1
This novel comonomer was synthesized in a two-step proce-
dure as given below and stored in freezer before use.
Step I: Excess 1-bromohexane (21.048 g, 127.5 mmol) was
added to a solution of 1-vinylimidazole (VIm, 10 g, 106.25
mmol) in dry tetrahydrofuran (100 mL) in a round bottom
flask fitted with condenser and the mixture was heated at
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8.02 (s, 1H, imidazole CHAN1), 9.3 (s, 1H, imidazole
NACHAN1). 13C NMR (400 MHz, d4-MeOD, d): 14.45, 23.74,
27.29, 27.29, 30.11, 30.96, 33.08, 51.28, 109.95, 120.78,
124.44, 129.82.
internal standard) were dissolved in methanol (1.5 mL) and
transferred to a 25 mL Schlenk tube (IPC1:AIBN 5 50:1).
The solution was then degassed with three freeze-pump-
thaw cycles using N2 and then heated on an oil bath at 70
ꢀC for 20 h. Aliquots were collected to periodically to deter-
1
Step II: Methanolic solution (15 mL) of 3-hexadecyl-1-vinyli-
midazolium bromide (C16VImBr, 10 g, 25 mmol) was added
dropwise to an aqueous solution (100 mL) of SSS (6.194 g,
30 mmol) and stirred at room temperature. Precipitation
was observed immediately and the reaction mixture was
stirred overnight. IPC2 was collected by filtration followed
by drying in a vacuum oven at room temperature to yield
white powder which was stored in a freezer before use.
Yield 11.91 g, 95%. The product was soluble in MeOH,
CHCl3, acetone, insoluble in water, THF and partially soluble
in DMF and ACN. 1H NMR (400 MHz, CDCl3, d): 0.86 (t, 3H,
-CH3), 1.19-1.24 (m, 26H, 133CH2), 1.77 (t, 2H, CH2), 4.20
(t, 2H, NACH2), 5.24-5.27 (m, 2H, 23vinyl CHCH2 of vinylimi-
dazole and styrene units), 5.72-5.86 (dd, 2H, imidazole and
styrene vinyl CH2), 6.64-6.71 (m, 1H, styrene vinyl CH), 7.30-
7.34 (t, 1H, imidazole vinyl CH) 7.37-7.39 (m, 3H, styrene
23CH and imidazole NCH), 7.69 (s, 1H, imidazole CHAN1),
7.82-7.84 (d, 2H, styrene 23CH), 10.24 (s, 1H, imidazole
NACHAN1). 13C NMR (400 MHz, CDCl3, d): 14.11, 22.68,
26.21, 28.99-30.12, 31.91, 50.39, 109.34, 115.11, 118.92,
122.47, 125.96, 126.29, 128.54, 136.13, 136.56. Anal. calcd.
For C29H46N2O3S: C 69.28, H 9.22, N 5.57, S 6.38, O 9.55%,
Found: C 69.09, H 9.18, N 5.52, S 6.35, O 9.40%.
mine the monomer conversion by H NMR spectroscopy. The
reaction became heterogeneous in after 2 h. Finally the reac-
tion was stopped by cooling to room temperature followed
by addition of excess chloroform to the reaction mixture.
The white powdery polyꢀmer CP1 was filtered off and dried
in a vacuum oven at 50 C overnight to yield 0.475 g (95%)
polymer measured gravimetrically.
Representative ATRP of IPC1
Monomer (IPC1, 0.5 g, 1.379 mmol), ethyl-2-bromoisobuty-
rate (EBiB, 5.38 mg, 0.028 mmol), N,N,N’,N”,N”-pentamethyl-
diethylenetriamine (PMDETA, 9.56 mg, 0.055 mmol) and
DMF (six drops, as internal standard) were dissolved in
methanol (1.5 mL). Separately a 25 mL Schlenk tube was
charged with CuBr (3.96 mg, 0.028 mmol) and purged with
N2. The earlier solution was then added to the Schlenk tube
and cooled immediately in a liquid N2 bath. The content
(IPC1:EBiB:CuBr:PMDETA 5 50:1:1:2) was degassed with
three freeze-pump-thaw cycles using N2 gas and then heated
on an oil bath at 70 ꢀC for 44 h. Aliquots were withdrawn
from the reaction mixture periodically to determine the
monomer conversion.
Representative RAFT Polymerization of IPC1
A solution monomer (IPC1, 0.5 g, 1.379 mmol), 2-cyano-2-
propyl benzodithioate (CPBD, 6.1 mg, 0.028 mmol), AIBN
(1.4 mg, 0.008 mmol) and DMF (six drops, as internal stand-
ard) in methanol (1.5 mL) was transferred to a 25 mL
Schlenk tube. The content (IPC1:CPBD:AIBN 5 50:1:0.3) was
then degassed with three freeze-pump-thaw cycles using N2
gas and then heated on an oil bath at 70 ꢀC for 44 h. Ali-
quots were withdrawn from the reaction mixture periodi-
cally to determine the monomer conversion by 1H NMR
spectroscopy. Finally the reaction was stopped by cooling to
room temperature followed by precipitation of polymer from
excess ether. The light pink powdery polymer CP3 was fil-
Synthesis of IPC3
This was synthesized by a modified method of Qiu et al.24
where 1-vinylimidazole (VIm, 2.001 g, 21.269 mmol) was
added to a dispersion of equimolar amount of 2-acrylamido-2-
methyl-1-propanesulfonic acid (4.408 g, 21.269 mmol) in
methanol (20 mL). A homogeneous solution was formed after
stirring for a while. The clear solution was stirred further for
4 h at room temperature and finally methanol was removed
in a rotary evaporator followed by high vacuum oven to yield
a clear viscous liquid. The product was stored in freezer
before use which finally produced white waxy solid. Yield
6.41g, ꢁ100%. The product was soluble in methanol, water,
partially soluble in chloroform and ethanol and insoluble in
ꢀ
tered off and dried in vacuum oven at 50 C overnight. Yield
350 mg (70%).
1
ACN. H NMR (400 MHz, d4-MeOD, d): 1.57 (s, 6H, acrylamide
23CH3), 3.21 (s, 2H, acrylamide CH2S), 5.44 (d, 1H, imidazole
vinyl CH2), 5.57 (d, 1H, acrylamide vinyl CH2), 5.92 (d, 1H, im-
idazole vinyl CH2), 6.08-6.23 (m, 2H, acrylamide vinyl CH and
CH2), 7.32 (m, imidazole vinyl CH), 7.66 (s, 1H, imidazole
NCH), 8.01(s, 1H, imidazole CHNH1), 9.19 (s, 1H, imidazole
NCH NH1). 13C NMR (400 MHz, d4-MeOD, d): 27.01, 53.48,
60.28, 109.19, 120.19, 121.97, 125.56, 129.91, 133.58, 135.87,
167.47. Anal. calcd. For C12H19N3O4S: C 47.83, H 6.35, N
13.94, S 10.64, O 21.24%; Found C 45.21, H 6.35, N 12.97, S
10.22, O 22.58%.
A part of polymer sample (150 mg) was dissolved in 0.2 M
NaBr (5 mL) and dialyzed against distilled water using
MWCO 3.5 K dialysis tubing at 50 ꢀC for 2 days. Polymer
from aqueous solution was finally collected by removing
water using high vacuum rotary evaporator at 60 ꢀC fol-
lowed by drying the sample in high vacuum oven at 60 ꢀC
overnight. Final polymer weight was ꢁ130 mg).
Representative Dispersion Polymerization of IPC2 in
Water
Monomer (IPC2, 0.5 g, 9.9531024 mol) was dispersed in
water (15 mL) in a 50 mL round bottom flask and the mix-
ture was purged with N2 for 20 min. Then the dispersion
was heated to 70 ꢀC on an oil bath to produce cloudy (and
foamy) solution. Aqueous solution of ammonium persulfate
Synthesis of Polymers
Representative FRP of IPC1
Monomer (IPC1, 0.5 g, 1.379 mmol), 2,20-azobisisobutyroni-
trile (AIBN, 4.52 mg, 0.028 mmol) and DMF (six drops, as
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(APS, 3.4 mg, N2 purged) was added and the polymerization
was continued for 4h. Polymer started precipitating after a
few minutes of the addition of APS. Finally the precipitated
polymer CP11 was filtered and dried in a vacuum oven to
yield white powder (yield 70%). The product was insoluble
in THF, DMF, ACN, acetone, toluene, 2,2,2-trifluoro ethanol
(TFE) and formamide.
12 mm, wt 19 mg, weight loss 5 4%) of swelled disc. For
comparison a disc of pure poly(MMA) of similar size was
produced and solubility behavior in THF was studied.
Synthesis of Polymer Network Between Poly(C6VImBr)
and Poly(SSS)
At first 3-hexyl-1-vinylimidazolium bromide (C6VImBr, 1 g,
3.858 mmol) was polymerized by AIBN (12.67 mg, 0.077
mmol) under N2 at 70 ꢀC in MeOH for 48 h. Polymer was
precipitated out from ether and dried in oven to yield 0.8 g
of poly(C6VImBr). Next, SSS (5 g, 24.25 mmol) was polymer-
Dispersion Copolymerization of MMA with IPC1 in Water
Monomer (IPC1, 0.1 g, 2.75831024 mol, 2.86 mol % of total
monomer) was dispersed in water (10 mL) in a 50 mL
round bottom flask and the mixture was purged with N2 for
20 min. The dispersion was heated to 70 ꢀC on an oil bath
to produce cloudy solution. Then the N2 purged methyl
methacrylate (MMA) (1 mL, 0.936 g, 9.34831023 mol, 97.14
mol %) was added and stirred at 500 rpm for 10 min. Aque-
ous solution of ammonium persulfate (APS, 10.67 mg, N2
purged, 0.5 mol % relative to monomers) was added and the
polymerization was continued. Polymer started precipitating
after a few minutes of the addition of APS and the reaction
was stopped after 4h. Finally the precipitated polymer CP12
was filtered and dried in a vacuum oven to yield white pow-
der (yield 600 mg, 58%). The product was insoluble in THF,
CHCl3, ACN, acetone, toluene, swelled in DMF but soluble in
2,2,2-trifluoro ethanol (TFE), 1,1,1,3,3,3-hexafluoroisopropa-
nol (HFiP) and 10% LiTFSI and LiPF6 solution of DMF
quickly. Found: C 59.27, H 7.78, N 1.45, S 1.64% (IPC1 unit
content in copolymer was ꢁ5.5 mol %).
ꢀ
ized in water under N2 at 70 C for 5.5 h using APS initiator
(111 mg, 0.486 mmol). Poly(SSS) (4.8 g) was precipitated
out from acetone and dried in oven. Then, a solution of pol-
y(C6VImBr) (108 mg, mmol, 0.417 mmol respect to repeat
unit) in 2:1 MeOH/water (5 mL) was added slowly to a
aqueous solution (20 mL) of poly(SSS) (85.92 mg, 0.417
mmol respect to repeat unit) at room temperature and
stirred for 2 h before heating at 90 ꢀC for 2 h. The white
precipitate product was collected by centrifuging the mixture
followed by drying in vacuum oven. Yield 120 mg (80%).
Found: C 57.8, H 7.26, N 8.48, S 7.58, O 17.92%.
RESULTS AND DISCUSSION
Synthesis and Characterization of IPCs
Vinylimidazole-based IPCs, IPC1 and IPC2, were synthesized
by a two-step process and IPC3 was synthesized by a one-
step process as shown in Scheme 1. For the synthesis of
novel IPC1 (Fig. 1), at first, 3-hexyl-1-vinylimidazolium bro-
mide (C6VImBr) was synthesized by the quaternization of 1-
vinylimidazole (VIm) with 1-hexyl bromide. Mixing of this
aqueous salt solution with the aqueous solution of SSS pro-
duced a homogeneous solution. However, the product IPC1
was extracted from the aqueous solution using CHCl3. The
salt formed as a byproduct, viz., NaBr remained in the aque-
ous solution. On removal of the organic phase, IPC1 was
obtained as a fine white powder. The presence of equimolar
quantities of components, that is, 3-hexyl-1-vinylimidazolium
Sample Preparation and Swelling Studies of CP12
CP12 (50 mg) was dissolved in 1 mL TFE. A part of this so-
lution was used to fill a disc shape Teflon die which when
dried (at room temperature for 24 h and then in oven at 50
ꢀC for 5 h) produced a disc shape polymer sample of 12 mm
diameter and ꢁ 1 mm thickness (weight 19.8 mg). This disc
was then added THF (2 mL) in a glass vial, stirred occasion-
ally and kept aside for 3 days. Finally the disc sample was
taken out and diameter and weight was measure before dry-
ing (diameter 15 mm, wt 40 mg) and after drying (diameter
SCHEME 1 Synthesis of different IPCs.
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proton at 9.29 ppm of C6VIm cation of IPC1 (Fig. 2) was due
to the acidic nature of this proton and possible deuterium
exchange of this proton with the NMR solvent, that is, CD3OD.
Higher intensity of N1@CHAN proton of VIm cation was
observed in lesser or nonexchangeable solvents like CDCl3
(see Supporting Information Fig. S1). 13C NMR spectrum of
this comonomer showed the expected signals (Fig. 2).
Similarly, 3-hexadecyl-1-vinylimidazolium bromide (C16VImBr)
was synthesized by the quarternization of VIm with 1-hexa-
decyl bromide (Scheme 1). Interestingly, mixing of the aque-
ous solution of both 3-hexdecayl-1-vinyl imidazolium bromide
(C16VImBr) and SSS instantly produced a white precipitate of
IPC2 (Fig. 1 and Scheme 1). Again the presence of equimolar
quantities of components, that is, 3-hexadecyl-1-vinylimidazo-
lium (C16VIm) cation and SS anion in IPC2 was confirmed by
NMR spectroscopy and elemental microanalysis. The signals
corresponding to CAH resonances of C16VIm cation shifted
upfield in 1H NMR spectra when the bromide (Br2) counter
anion of C16VImBr was replaced by the 4-styrenesulfonate
FIGURE 1 IPCs used for this study.
(C6VIm) cation and 4-styrenesulfonate (SS) anion in IPC1
was confirmed by NMR spectroscopy (Fig. 2) and elemental
microanalysis. The characteristic vinyl @CAH resonances
were observed at 5.43, 5.89, and 7.23 ppm for C6VIm cation
and at 5.34, 5.89, and 6.78 ppm for SS anion in nearly 1:1 ra-
tio in the 1H NMR spectrum of IPC1 along with all other
expected signals (Fig. 2). The reduced intensity of N1@CHAN
FIGURE 2 (i) 1H NMR and (ii) 13C NMR spectra of IPC1 (in CD3OD).
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(SS) counter anion to produce IPC2 as seen in Supporting In-
formation Figure S2 due to the change in interaction between
cation and anion.
Ionic Liquid Behavior of IPCs
The ion pair comonomers IPC1–3, have also been studied by
DSC under N2 atmosphere. IPCs, IPC1–3, were found to be
high temperature ionic liquids with melting temperatures
(Tm) of 69.7 ꢀC, 94.6 ꢀC, and 111.9 ꢀC,ꢀrespectiveꢀly. Heating
of representative IPC2 in DSC from 20 C to 270 C showed
(Fig. 4) one endothermic transition corresponding to the
melting of IPC2 and two exothermic transitions due to poly-
merizations. Any exothermic transition corresponding to sol-
idification or crystallization was not observed in the cooling
cycle during this experiment. This was due to the polymer-
ization of the comonomer during the heating cycle (owing to
heating to high temperature). However, both solid-to-liquid
and liquid-to-solid transitions of IPC2 were observed during
heating and cooling cycles, respectively, when the DSC
experiment was performed from 20 ꢀC to 110 ꢀC thereby
avoiding the polymerization (Fig. 4).
The other comonomer IPC3 (Fig. 1), was synthesized by mixing
methanolic solution of VIm with an equimolar methanolic dis-
persion of 2-acrylamido-2-methyl-1-propanesulfonic acid
(Scheme 1). The resulting solution was homogeneous and
yielded IPC3 as colorless, highly viscous mass on removal of
methanol under vacuum. The product was finally transformed
to a waxy solid on storing in a freezer. IPC3 was not simply a
physical mixture of VIm and 2-acrylamido-2-methyl-1-propane-
sulfonic acid. Instead it was in ionic form as confirmed by 1H
1
NMR spectroscopy (Fig. 3). The H NMR signals corresponding
to CAH resonances of VIm appearing at 4.97, 5.50, 7.14, 7.48,
and 7.90 ppm were shifted downfield to 5.47, 5.97, 7.32, 7.67,
and 8.02 ppm, respectively, and the largest shift was observed
for the N1@CHAN proton from 7.04 to 9.21 ppm confirming
the protonation of VIm unit to produce VImH cation. However,
proton signals of 2-acrylamido-2-methyl-1-propanesulfonate
anion did not shift due to its nonconjugated nature.
Polymerization of IPCs
Polymerization of IPC1–3 by FRP, ATRP, and RAFT-mediated po-
lymerization were performed (targeted degree of polymerization
FIGURE 3 Overlay 1H NMR spectra of IPC3 and its precursors (in CD3OD).
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homogeneous throughout the course of polymerization.
Finally, the copolymer was separated by precipitating in ether
to produce pink powder (Supporting Information Fig. S3).
Interestingly, unlike CP1 formed by FRP of IPC1, the copoly-
mer CP3 produced by RAFT from IPC1 was soluble in some
common organic solvents including methanol and also in 0.1
N aqueous solution of NaNO3 (Table 1). Thus, it was possible
to characterize CP3 by aqueous GPC [Fig. 5(a)] and also by
NMR spectroscopy. Although IPC1 composed of equimolar
mixture of 1:1 3-hexyl-1-vinylimidazolium (C6VIm) cation and
styrenesulfonate (SS) anion, the copolymer CP3, was formed
by 37% and 79% of conversion of cations and anions, respec-
tively. This was possibly due to the difference in chain trans-
fer and reinitiation capability of polymer chains ending with
C6VIm based repeat unit in comparison to the chain end ter-
minating with SS based repeat unit. However, this has created
a charge imbalance within the polymer chain and therefore
the negative charge of about 42% excess anionic SS unit was
to be neutralized by the nonpolymerized or monomeric
FIGURE 4 Overlay DSC plots of IPC2 for different heating-cool-
ing cycles: (a) 20–270 ꢀC and (b) 20–110 ꢀC.
1
C6VIm cations. This was indeed observed in the H NMR spec-
trum of CP3. Repeated dialysis of copolymer CP3 even in the
presence of NaBr as well as at higher temperatures (50 ꢀC)
did not remove monomeric C6VIm cations completely from
was 50) in methanol and the time dependent conversions of
individual ionic unit of IPCs were determined by H NMR spec-
1
troscopic analysis using either CDCl3 or preferably CD3OD as
solvent. Controlled radical polymerization techniques were used
here as the polymerization techniques like ATRP,19,20 SET-LRP,21
and RAFT20,22 can be expected to provide greater control over
structure of polymers synthesized and also for the purpose of
verifying the suitability of these techniques for surface initiated
polymerizations23 of IPCs.
1
the polymer as was observed in the H NMR spectrum of dia-
lyzed copolymer (Fig. 6). This may be due to the higher affin-
ity of anionic SS derived repeat units to C6VIm cations over
the inorganic Na1 cations.
The improved solubility of CP3 may be attributed to various
factors such as greater control over the polymer structure
caused by RAFT polymerization, due to the weaker inter-
chain interactions among the inherently charge imbalanced
polymer chains and because of the presence of monomeric
C6VIm cations to the polymer chains (Scheme 2) through
ionic interactions, and so forth. The gel permeation chromat-
ogram of CP3 was monomodal, molecular weight (Mn,GPC) of
the copolymer matched fairly well with the theoretically esti-
mated molecular weight (Mn,Theo) and the polydispersity
index (PDI) was also narrow (1.23) [Fig. 5(a) and Table 1].
The FRP of IPC1 using 2,20-azobisisobutyronitrile (AIBN) as
initiator was heterogeneous and yielded a powdery white co-
polymer (CP1 in Table 1). Both the final monomer conver-
sion and polymer yield were high (ꢁ95%). Characterization
of this copolymer was difficult because of the insolubility of
CP1 in all common organic solvents tested including highly
polar solvents like 2,2,2-trifluoroethanol (TFE) and formam-
ide as well as in different aqueous solutions (Table 1). The
highly insoluble nature of copolymer was most probably due
to the prevalence of very strong intermolecular interactions
of cationic and anionic pendant groups of CP1.
Monomer conversion in FRP of IPC2 was high (>85%), and
the corresponding polymer was completely insoluble (CP4 in
Table 1). Several of our attempts to make soluble copolymer
by FRP of IPC2 by using different polymerization solvents like
N,N-dimethylformamide (DMF), 1,2-dichloroethane, ethanol, or
by dispersion polymerization in water did not succeed. ATRP
of IPC2 even at higher temperature failed to produce any
polymer (Table 1) again possibly due to the catalyst poisoning
by the VIm moiety as well as poor reactivity of Vim under
ATRP. Again no successful ATRP or SET-LRP has been
reported in the literature for its parent monomer consisting
of 3-hexadecyl-1-vinylimidazolium (C16VIm) species. RAFT-
mediated polymerization of IPC2 was, however, homogeneous
and yielded pink-colored copolymer with high conversions
when precipitated from excess MeOH at room temperature.
The conversion of C16VIm cation and SS anion were not same.
As previously noted for the RAFT polymerization of IPC1, the
charge imbalance in the polymer of IPC2, although low, was
nevertheless noticed. The conversions of C16VIm and SS ions
Both monomer conversion and copolymer yield from the
ATRP of IPC1 using traditional ethyl-2-bromoisobutyrate
(EBiB) initiator, CuBr catalyst and N,N,N0,N00,N00-pentamethyl-
diethylenetriamine (PMDETA) ligand were low (below 12%,
CP2 in Table 1). The polymerization reaction was homogene-
ous, and the resultant polymer was difficult to isolate in pure
form. Hence, characterization by GPC was not possible. This
was no surprise as no successful ATRP or SET-LRP has been
reported in the literature even for the parent 3-hexyl-1-vinyli-
midazolium (C6VIm) based monomers most probably because
of the catalyst poising by the VIm moiety. However moderate
conversions and yield were observed in the RAFT-mediated
polymerization of this comonomer (Scheme 2 and Table 1)
using AIBN as initiator and 2-cyano-2-propyl benzodithioate
(CPBD) as chain transfer agent. The rate of polymerization
was slower than FRP as expected, and the reaction was
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SCHEME 2 Synthesis of CP3 by RAFT polymerization of IPC1 (RAFT agent derived end functional groups have been omitted for
simplicity).
were 64.8% and 84%, respectively. The copolymer CP6
showed surprising solubility behavior. CP6 was insoluble in
DMF, THF, CHCl3, and MeOH. However, it was soluble in mix-
ture of DMF/MeOH, THF/MeOH, and CHCl3/MeOH. The
improved solubility of CP6 over CP4 was most probably
attributed to both controlled nature of the polymer structure
and the ionically attached monomeric C16VIm cations to the
polymer chains to neutralize the excessive negative charge
present in the polymer chain. The presence of ionically
attached, nonpolymerized C16VIm cations to the copolymer
insolubility of CP6 in a suitable solvent which is used in GPC
as eluant.
Vinylimidazole (VIm) and acrylamide-based asymmetric
comonomer IPC3 (Fig. 1) was polymerized in the same way
as described before for IPC1 and IPC2 (Table 1). However,
FRP of IPC3 in MeOH proceeded homogeneously. The mono-
mer conversions were high but unequal. The copolymer CP7
was soluble in a range of solvents and hence was character-
ized by 1H NMR spectroscopy (Supporting Information Fig.
S4) as well as GPC analysis [Fig. 5(b)].
1
chain was again observed in the H NMR spectrum of dialyzed
CP6. The copolymer CP6 was dissolved in a mixture of CDCl3
and CD3OD for NMR spectroscopic analysis. The molecular
weight of copolymer CP6 could not be determined due to the
The dialyzed product of this copolymer still revealed the
presence of some monomer, VImH cations ionically bonded
with the copolymer to neutralize the excessive anions
derived from acrylamide comonomer of IPC3. It may be
noted here that unlike copolymers CP1, CP4, and CP7 exists
in its protic form and thus exhibits different solubility
characteristics.
Similar to IPC1 and IPC2, IPC3 also could not be polymer-
ized under ATRP. The lack of polymerization under ATRP
may be due to the presence of exchangeable protons in the
polymerization system. It is well known that acidic mono-
mers cannot be directly polymerized under ATRP. However,
RAFT-mediated polymerization of IPC3 proceeded with mod-
erate conversions and yield. The molecular weight of the col-
ored (Supporting Information Fig. S3) copolymer CP9
matched fairly well with the theoretically estimated value
and the PDI was narrower than the polymer obtained by
FRP (i.e., CP7) [Fig. 5(b) and Table 1].
One common feature of all IPCs during polymerization has
been that conversion of anionic comonomer of the ion pair
monomer was invariably higher. This is most likely due to a,
b-unsaturated (i.e., conjugated) nature of anionic comonomer
because of which its reactivity is far greater than that of the
cationic counterpart derived from VIm.
Thermal Properties of IPCs and CP1
Thermal stability of all asymmetric IPCs, IPC1–3, and a rep-
resentative copolymer produced from IPC1 by FRP, that is,
CP1 were determined by TGA under nitrogen atmosphere
FIGURE 5 Aqueous GPC chromatogram of (a) CP3 and (b)
overlay chromatograms of CP7 (red) and CP9 (green).
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FIGURE 6 1H NMR spectrum of CP3 in CD3OD after dialysis.
(Fig. 7). In general, due to the hygroscopic nature of these
comonomers and polymer, weight loss was invariably
observed below 100 C. Comonomers IPC1 and IPC2, which
neutralized copolymers as confirmed by monomer conver-
sion as well as NMR spectroscopic analysis clearly points to
the prevalence of structure proposed in Scenario C as for
Scenario B to prevail, equal conversion of IPCs is a prerequi-
site. This scenario here is unique caused by the unusual
reactivity of vinyl monomers which are the components of
IPCs unlike previously reported polyampholytes6,8–11 synthe-
sized from symmetrical IPCs with similar reactivity.
ꢀ
were made of C6VIm or C16VIm and SS ions, respectively,
were stable up to 300 ꢀC. However, IPC3, which was
obtained by combining VImH and acrylamide-based ions,
ꢀ
was stable only up to 200 C. The residual weight for all the
materials analyzed was 8%–10%, which is most probably
due to the char formation of heteroatom containing
materials.
The insoluble nature of copolymers obtained by FRP of IPC1
and IPC2 (i.e., CP1 and CP4, respectively, in Table 1) even in
very polar organic solvents as well as in concentrated salt
solutions supports the strong interchain interaction, although
may not be 100%, combined with the formation of high mo-
lecular weight polymers. The lack of solubilizing behavior of
these polymers due to interchain interactions was partially
supported by the synthesis of fully inter chain ionically
crosslinked polymer prepared according to Scheme 4. The
homopolymers of C6VImBr and SSS were mixed in equimolar
quantities to instantly produce highly insoluble network co-
polymer of poly(C6VIm) and poly(SS) (Scheme 4).
Intra- versus Intermolecular Interactions in Copolymer
Produced by FRP of IPCs and their Solubility Behavior
Based on electrostatic interactions, ionic interactions of
copolymers synthesized from IPCs can be expected to be of
intra chain and inter chain in nature (Scenario A and B,
respectively, of Scheme 3) or more probably mixed interac-
tions (Scenario C of Scheme 3). The formation of partially
Dispersion Polymerization of IPCs and Preparation of
Ionically Crosslinked PMMA
IPC1 and IPC2 were insoluble in water at room temperature
but were sparingly soluble at high temperatures. The cloudy
and foaming solution of these comonomers was polymerized
using ammonium persulfate (APS) initiator to observe any
effect on the solubility of polymer produced. Due to the
foaming nature IPCs it can be considered as polymerizable
surfactants (Supporting Information Fig. S5). However, dur-
ing the course of polymerization the polymer was precipi-
tated out as white solid from the reaction mixture. The
polymers thus formed, CP10 and CP11 (Table 2), were
FIGURE 7 TGA of IPC1, IPC3, and CP1.
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SCHEME 3 Possible polymerization products of IPCs.
SCHEME 4 Interchain ionic crosslinking.
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TABLE 2 Dispersion Copolymerization of IPCs
Polymer
Yielda (%)
Comonomer
Code
Solubility of Polymerb
IPC1
CP10
CP11
CP12
72
70
58
Insoluble
Insoluble
IPC2
IPC1 and MMA
Insoluble in THF, acetone, CHCl3, swelled in DMF,
soluble in TFE and in DMF containing Li1 salts.
a
All polymerizations were performed in water under N2 and at 70 ꢀC for
4 h using ammonium persulfate (APS) initiator with monomer:initiator
50:1 ratio.
Determined gravimetrically.
b
Solvents used for the solubility tests were CHCl3, ACN, THF, DMF, ace-
tone, MeOH, H2O, 20% NaCl in H2O, TFE.
insoluble in all solvents tested which was similar to the solu-
bility behavior of copolymer CP1 and CP2 discussed before.
fracture behavior, and so forth, in comparison to their linear
analogs such as PMMA,25(a) PS25(b), and so forth. Therefore, a
small quantity of IPC1 (2.86 mol %) was copolymerized
with excess MMA (97.14 mol %) in water by dispersion po-
lymerization technique using APS initiator (Fig. 8). The white
powdery copolymer CP12 contained about 5.5 mol % of
IPC1 as determined by nitrogen content in elemental analy-
sis and was mainly composed of MMA (ꢁ94.5 mol %).
Unlike the homopolymer PMMA, this copolymer CP12 [poly
(MMA-co-IPC1)] was insoluble in THF, acetone, CHCl3, and
only swelled in DMF. It may be noted that swelling is a
The strong inter chain interaction of copolymer produced
from IPC1 was exploited to synthesize ionically crosslinked
PMMA by copolymerizing MMA with small quantity of IPC1.
The interaction between C6Vim cations and SS anions
derived from two or more polymer chains can be expected
to result in network formation. This kind of ionically cross-
linked materials were reported previously to have different
physical properties, for example, stiffness, fatigue strength,
FIGURE 8 Synthesis of ionically crosslinked PMMA, that is, CP12 by aqueous dispersion polymerization and swelling study (in
THF) of a disc shape sample produced using the copolymer.
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ACKNOWLEDGMENTS
tendency commonly associated with networked polymers.
However, this copolymer was soluble in 10% lithium salt
(like LITFSI or LiBF4) solution of DMF or DMSO. Copolymer
CP12 was also soluble in strongly H-bonding solvents like
2,2,2-tifluoroethanol (TFE) and 1,1,13,3,3-hexafluoro isopro-
panol (HFiP). This unusual solubility behavior of this copoly-
mer mainly composed of MMA is most likely due to the
intermolecular ionic interactions as depicted below (Fig. 8).
The solubility behavior in salt solutions as well as strongly
H-bonding solvents further confirm the existence of weakly
networked structures where the network junctions are non-
covalent in nature and thus can be disrupted by ionic as
well as H-bonding interactions. The disruption of ionic inter-
action occurs due to the effective solvation of ions by these
solvents. Several other experiments confirmed that poly(-
MMA-co-IPC1) with < 3 mol % of IPC1 content was insolu-
ble in THF and CHCl3 but soluble in DMF. However,
poly(MMA-co-IPC1) with 20 mol % of IPC1 content was in-
soluble in all organic solvent including TFE and HFiP.
This work was funded by Agency for Science, Technology and
Research (A*STAR), Singapore under Innovative Marine Anti-
fouling Solutions (IMAS) for high value applications program.
The authors thank Mr. Zhao Wenguang and Ms. Foo Ming Choo
for their assistance during synthesis and characterization of
monomers and polymers.
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